Springs and torsion bars made from formed steel wire are known in the prior art in a multitude of embodiments. Torsion bars are also referred to, for example, as torque rod springs, stabilization torque rods or torsion bar springs. Steel springs and torsion bar strings are used especially in motor vehicles, where steel springs are used, for example, to absorb road unevenness in shock absorber systems, and torsion bar springs to provide stabilization against tilting and distortion of the chassis, especially on motor vehicle cornering, on motor vehicle journeys over varying road surfaces and in the event of road unevenness. The shaping of the steel wire to give springs and torsion bars can be effected by a cold and/or hot forming method. Prior to this shaping, the steel wire can undergo various preparation steps which affect the spring and strength properties. For example, the spring steel used for production of a steel spring and/or torsion bar spring is subjected to a thermomechanical forming (TMF) operation, in order to increase its strength and toughness usable for construction purposes and to improve further specific use properties of a material. For instance, springs and/or torsion bars having high strength(s) can be produced with lower material input and hence low weight and material costs. The prior art discloses a number of different methods which comprise a thermal treatment and then a forming operation. In the case of cold forming, the formability of the steel wire is limited, since the toughness and formability thereof decreases as a result of cold solidification with an increasing degree of forming.
In the mass production of hot-formed helical springs, the TMF is already used in the form of a skew rolling process, but here only on prefabricated individualized spring rods. Such a process is disclosed in DE 103 15 418 B3. The TMF is effected on the spring rod by a one-stage skew rolling process directly prior to the hot winding of the helical springs. The hot-formed spring is quenched in oil, which results in a martensitic structure. DE 198 39 383 C2 describes a process for thermomechanical treatment of steel for torsion-stressed spring elements. A starting material is rapidly heated to a temperature of 1080° C. and austenitized. Subsequently, the starting material is subjected to a TMF, which achieves recrystallization. Subsequently, without intermediate cooling, the starting material is hardened by quenching.
This process is conducted in an integral manufacturing line in which all steps are conducted from the TMF up to the quenching. The direct concatenation of thermomechanical forming and tempering which is thus required results in the following disadvantages:                1. Changes in the length of the wire resulting from the thermomechanical forming, usually rolling, have a direct effect on the process parameters of the immediately subsequent hot forming and tempering.        2. The process times and temperatures of the thermomechanical forming, the hot winding and the tempering have to be matched to one another, which is difficult to implement in terms of process technology. This is because a preferred temperature for the thermomechanical forming is one just above the austenitization temperature of the wire material, while heating to a much higher temperature is advantageous for the hot forming and the tempering.        3. Between the TMF and the hot winding, further processing steps on the rod are required (for example cutting to an exact length), which extends the period before quench hardening. Therefore, the rod is kept at a very high temperature for a certain time, which can result in adverse changes in structure, for example grain growth and decarburization.        4. The TMF and the hot winding apparatus have different run times for each spring rod. The throughput of the manufacturing line would thus be defined by the slowest process component; the quicker process components are thus working not at capacity and therefore uneconomically.        5. A shutdown in any process component (for example for maintenance or because of a fault) shuts down the entire manufacturing line.        6. For every winding system, a separate TMF unit has to be kept ready. In the case of a multitude of steel springs to be manufactured simultaneously, this means a corresponding number of TMF units.        7. The processing of spring rods with non-constant wire diameter is currently possible with an integral manufacturing line only with a considerable degree of control complexity, if at all.        8. The one-stage skew rolling operation employed in the manufacturing of springs for thermomechanical forming (corresponding to the abovementioned DE 103 15 418 B3) leads to rotation of the wire about its longitudinal axis at a speed of 400 rpm or more. This can be conducted with individualized spring wires, but not in the case of a continuous wire. It is already known that a two-stage caliber rolling operation can be used instead of skew rolling. However, the above disadvantages 1 to 7 also exist when caliber rolling is employed.        